What is the full word for blurb?

There isn't really a "full word" for blurb. It's a word in its own right, with no longer or more formal synonym.

However, depending on the context, you might use other words to describe the same thing:

* Synopsis: A brief summary of a book, film, etc.

* Summary: A brief overview of something.

* Description: A detailed account of something.

* Logline: A one-sentence summary of a story.

* Teaser: A short piece of writing designed to pique interest.

Ultimately, the best word depends on the specific situation and what you're trying to convey.